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What is levothyroxine?
Levothyroxine is a replacement for a hormone that is normally produced by your thyroid gland to regulate the body's energy and metabolism. Levothyroxine is given when the thyroid does not produce enough of this hormone on its own. Levothyroxine treats hypothyroidism (low thyroid hormone). Levothyroxine is also used to treat or prevent goiter (enlarged thyroid gland), which can be caused by hormone imbalances, radiation treatment, surgery, or cancer. Levothyroxine should not be used to treat obesity or weight problems.

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What are the possible side effects of levothyroxine?

Stop using levothyroxine and get emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.

Call your doctor at once if you have any of these serious side effects:

• headache;

• sleep problems (insomnia);

• nervous or irritable feeling;

• fever, hot flashes, sweating;

• changes in your menstrual periods;

• appetite changes, weight changes;

Less serious side effects may include mild hair loss.

This is not a complete list of side effects and others may occur. Tell your doctor about any unusual or bothersome side effect.

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How should I take levothyroxine?
Take levothyroxine exactly as your doctor has prescribed it for you. Do not use more of the medication than recommended. Do not take levothyroxine for longer than your doctor has prescribed. Your doctor may occasionally change your dose to make sure you get the best results from this medication. It is very important to take levothyroxine with a full glass (8 ounces) of water. The levothyroxine tablet can dissolve very quickly and swell in the throat, possibly causing choking or gagging. Take this medicine on an empty stomach, 30 minutes before eating. Levothyroxine is usually taken in the morning. Follow your doctor's dosing instructions and try to take this medication at the same time each day.

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What is the most important information I should know about levothyroxine?
Since thyroid hormone occurs naturally in the body, almost anyone can take levothyroxine. You should not use this medication if you have had a heart attack, a thyroid disorder called thyrotoxicosis, or an adrenal gland problem that is not controlled by treatment. Before taking this medication, tell your doctor if you have heart disease, coronary artery disease, anemia (lack of red blood cells), diabetes, problems with your pituitary or adrenal glands, or a history of blood clots. If you use insulin or take diabetes medicine by mouth, ask your doctor if your dose needs to be changed when you start using levothyroxine.

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What happens if I miss a dose?
Take the missed dose as soon as you remember. If it is almost time for your next dose, skip the missed dose and take the medicine at the next regularly scheduled time. Do not take extra medicine to make up the missed dose.

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What other drugs will affect levothyroxine?
The following drugs may cause medical problems if you use them with levothyroxine: lithium, amiodarone, or antidepressants. Tell your doctor if you have recently received radiation therapy with iodine (such as I-131). This list is not complete and there are many other drugs that can interact with levothyroxine. Tell your doctor about all the prescription and over-the-counter medications you use. This includes vitamins, minerals, herbal products, and drugs prescribed by other doctors. Do not start using a new medication without telling your doctor.

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Can I take this if I am pregnant or trying to get pregnant or if I am breastfeeding?

Levothyroxine has been assigned to pregnancy category A by the FDA. Levothyroxine is a naturally occurring hormone, normally present in both maternal and fetal circulation. Levothyroxine, in replacement doses, is not expected to adversely affect the fetus. On the contrary, hypothyroidism may lead to poor pregnancy outcome. Thyroid replacement therapy should be maintained during pregnancy.

The Collaborative Perinatal Project monitored 50,282 mother-child pairs, of whom 537 were exposed to levothyroxine or thyroid during the first trimester. The standardized relative risk for malformations was 1.19 compared to a relative risk of 1.05 in the normal population. The standardized relative risks for cardiovascular malformations, polydactyly in Blacks, and Down syndrome were 1.61, 2.42, and 2.36, respectively. However, as the total number of malformations is small and because the contribution of thyroid disease itself cannot be excluded, the true risk of thyroid hormone use in the first trimester cannot be established based on these data. The extent to which levothyroxine is transferred from mother to fetus is controversial. Many early studies concluded only negligible amounts of levothyroxine are transferred to the fetus. However, a recent study evaluated maternal-fetal transfer of levothyroxine based on data obtained from 25 infants with a complete inability to form levothyroxine. Based on neonatal serum levothyroxine levels and an associated elimination half-life of 3.5 days, the authors estimated levothyroxine levels in cord blood to range from 35 to 70 nmol/L (normal T4, 80-170 nmol/L). As the infants were unable to synthesize levothyroxine, it was concluded that all levothyroxine present must have been of maternal origin. In the same study, 15 infants with thyroid agenesis were also evaluated. Serum concentrations of levothyroxine as well as elimination half-life were similar to those obtained from the infants with an inability to form levothyroxine. Again, these data support maternal transfer of levothyroxine late in pregnancy. TSH levels should be closely monitored during pregnancy. There is some evidence that hypothyroid women require more thyroxine during pregnancy.

Levothyroxine is excreted into human milk in small amounts. Levothyroxine, in replacement doses, is not expected to cause adverse effects in the nursing infant. The manufacturer recommends that caution be used when administering levothyroxine to nursing women. However, adequate replacement doses of levothyroxine are needed to maintain normal lactation.

In one study, levothyroxine levels in 70 milk samples from 20 euthyroid women 17 to 39 days after delivery were determined by gas chromatography-mass spectrometry (GCMS). Levothyroxine was present in milk in concentrations less than 4 ng/mL. The authors suggested that, at most, a breast-fed infant would ingest approximately 10% of the recommended dose for hypothyroid infants. The majority of other studies have used radio immunoassay (RIA), a less specific assay, to measure thyroid hormone concentrations in human milk. Such studies have yielded significantly variable results. Earlier studies concluded that levothyroxine and triiodothyronine were present in sufficient quantities to treat a hypothyroid infant. However, most subsequent studies have concluded otherwise. In addition, the presence of thyroid hormone in breast milk does not appear to interfere with neonatal thyroid screening.

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Who should NOT use this medication?

• Untreated subclinical (suppressed serum TSH concentrations with normal T₃ [triiodothyronine] and T₄ concentrations) or overt thyrotoxicosis of any etiology.
• AMI.
• Untreated adrenal insufficiency.
• Known hypersensitivity to any ingredient in the formulation. (See Sensitivity Reactions under Cautions.)

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What happens if I overdose?
Seek emergency medical attention if you think you have used too much of this medicine. Overdose symptoms may include chest pain, pounding heartbeat, shortness of breath, tremor, shortness of breath, leg cramps, confusion, vomiting, diarrhea, or seizures.

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What should I discuss with my healthcare provider before taking levothyroxine?
Since thyroid hormone occurs naturally in the body, almost anyone can take levothyroxine. You should not use this medication if you have had a heart attack, a thyroid disorder called thyrotoxicosis, or an adrenal gland problem that is not controlled by treatment. Before taking this medication, tell your doctor if you have heart disease, coronary artery disease, anemia (lack of red blood cells), diabetes, problems with your pituitary or adrenal glands, or a history of blood clots. If you use insulin or take diabetes medicine by mouth, ask your doctor if your dose needs to be changed when you start using levothyroxine.

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Can I stop taking the medication if I feel better?
As a general rule, you should always take your medications exactly as prescribed and do not change the dosage or stop taking the medication without first discussing it with your healthcare provider.

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I am on so many medications; do I have to take them all?
This is called polypharmacy—many different medications being used at the same time by one person. Sometimes, being on multiple medications is acceptable and appropriate but at other times it may be problematic. If you are receiving your medications from multiple physicians you need to ensure that they all know what medications you are taking. The best way to do this is to make a list of all the medications you are currently using, including all nutritional supplements, homeopathic remedies, vitamins and over-the-counter drugs (if possible, also include all the diseases you have been diagnosed with). Give a copy to every doctor who takes care of you so they have it on file, this way they can avoid duplicating medications and perhaps even try to consolidate some. After every doctor's visit remember to update the list accordingly. Also, as much as you possibly can, try to use the same pharmacy to fill all your prescriptions, this way any potential drug interactions can be caught and averted.

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Where can I get more information?
More Information

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